The present invention relates to a method of forming a cavity structure having a thin bottom base plate and a predetermined oddly shaped hollowed section, which is made from metal plate and suitable for chassises or base members of micro machines, portable information equipments and so forth, and particularly to a forming method of a thin bottom base plate at the base of the hollowed section while preventing the hardening of the work piece.
Through the development of information technology, devices and equipments for office automation and/or portable information equipments have been changing into the form of micro machines due to reduction in their size and weight. Accordingly, the chassis and the base members for use in such micro machines are required to be more precise and miniaturized.
FIG. 3 illustrates one example of a base member for use in the one of above-mentioned micro-machines. For an instance, in the base member 1 having thickness from 3 mm to 5 mm, oddly shaped hollowed section 2 is formed in one side, and a bottom base plate 3 is formed so as to have the thickness as thin as about 0.2 mm at the bottom of the hollowed section 2. Thus, as a whole, the base member 1 is formed as a cavity structure. Further, in the hollowed section 2, there exists parts supporting means such as a supporting pole 4, a pedestal portion 5 and so forth for arranging and supporting electrical and/or mechanical parts not illustrated. The parts supporting means are formed integral with the bottom base plate 3 so as to have different heights from the bottom base plate 3 to their top. As mentioned above, such a base member of a micro-machine as a whole is required to be formed thin in order to minimize its size while enlarging its capacity for receipting the parts. Therefore, it becomes an important issue to form the bottom base plate 3 in the hollowed section 2 as thin as possible.
As a forming method of such a base member 1, it is generally known that forging processing by means of a press punching by a press and processing performed by die-casting molding are applied.
The forging processing by the use of the press is performed as following. A metal plate with approximately the same thickness as the base member 1 is placed on a die and is pressed by the punch (not illustrated) from one side thereof. The punch has substantially the same oddly shape as the hollowed section 2 to form the hollowed section 2 into the oddly shape. On the other hand, die-casting molding is performed in such manner that molten metal of aluminum or so forth is injected into a die-casting mold having the same shape as the base member 1.
However, in case of the forging processing by the press, the quantity of metal corresponding to the hollowed section 2 shifts to its periphery, and it lead to curling of the base member 1, as illustrated in FIG. 7(A), due to stress at the time of the shifting of the metal. Such remarkable deterioration of flatness is a serious problem for the base member 1. In order to cure this curling, it is necessary to execute additional processing such as flat press. However, it is almost impossible to cure completely the curling caused by such an internal stress. Further, there is another problem that it is impossible to form the thickness t1 of the bottom base plate 3 as thin and precise as desired because there exist a limit that the bottom base plate 3 cannot be pressed by the pressure more than the critical pressure due to the hardening of the bottom base plate 3 occurs at the press processing.
On the other hand, in the case of the method by means of die casting molding, as illustrated in FIG. 7(B), there is a problem that a hole 3a is formed because molten metal such as aluminum or so forth easily flows into the area of the mold having large thickness but is difficult to flow in the bottom base plate 3, that is to say, such area having thin thickness t1xe2x80x2 where is intended to be formed as thin as about 0.2 mm or less. Furthermore, there is another problem that scale-down and/or strain occurs and, accordingly, dimension accuracy of the base member 1 deteriorates in the course where molten metal of high temperature cool down. Thus, it becomes necessary to perform re-processing such as grinding and so forth in order to obtain flatness with high precision. However, since the bottom base plate 3 of the hollowed section 2 is too thin to carry out the re-processing such as grinding and so forth so that the base member 1 with high precision cannot be obtained.
The present invention has been made in order to improve the above-mentioned problems in the conventional method. An object of the present invention is to provide a method for forming a cavity structure made of a metal plate in high precision and in low cost. The cavity structure has a cavity suitable for installing equipment parts of micro machines and the bottom surface of the cavity has supporting means for support the equipment parts.
The method for forming a cavity structure provided with a thin bottom plate is comprises the following steps. The first step is to deform plastically a part of a metal plate so as to form a cavity on one surface of a metal plate wherein the cavity has an oddly bottom surface provided with subsections being different from each other in depth. Simultaneously a protrusion is formed on the other surface of the metal plate by shifting an amount of metal corresponding to the cavity into the protrusion wherein the protrusion has substantially similar figure to the cavity. Then a coupling section, which keeps the protrusion integral with the metal plate, is formed so as to make the protrusion smaller than the cavity. And the protrusion is removed from the metal plate so as to make the other surface of the metal plate flat and to form the bottom plate of the cavity thin while remaining the coupling section.
The various features of novelty which characterize the invention are pointed out particularly in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.